Why is my diff backup so large?

I've just done a full backup on my Boss' laptop using True Image Home 10. The full backup came in at around 14gb. Now, two days later, I've gone to run a differential backup, but it is telling me the estimated backup size is about 14gb. Why is this? This is the same experience I had when I ran an incremental backup; that file size was slightly larger than the base image!

I did not do a defrag on the machine. I made the base image, running it to an external USB HD. Two days later I ran an incremantal backup, and like I said the filesize was actually slightly larger than the base. I went Acronis' website and researched backups, and the differences between them. Decided to see what would happen with the differential, and it said the filesize would be about the same as the incremental.

There is nothing else running. It's actually a totally clean install of XP Pro, was about 5 days old when I made the image.

He works almost exclusively from home. I was hoping to get it set up so that I could keep the base image at the company, and every night have his differential backup run over the VPN. That will entirely depend of course on what the filesize ends up being on average.

Perhaps the second backup was also a full baclup and not a differential as you intended. Use the Mount Image wizard to select the backup file and see if it's identified as "Volume 2 of Multivolume image".